minor setup corrections
Major updates to ComboBox:
- Essentially a graphical interface to dict; all items have text and value
- Assigns previously-selected text after list is cleared and repopulated
- Get, set current value
PlotNode control widget now displays combo box to let user select plots to connect to
Flowchart example updated to use this feature.
fixes for python3
Merge branch 'flowchart_plotnode_menu' into develop